The present invention relates to wire dot print heads, and in particular, to wire dot print heads containing print levers which are rotatably mounted therein.
Wire dot print heads utilizing print levers are known in the art. Such levers include a print lever having a print wire fixed at one end. A drive coil is positioned adjacent the print lever so that when a pulse current is applied to the drive coil, the print lever rotates causing the print wire projecting from the print lever to print on a recording paper. Additionally, a multistage printer is known in the art from applicant's Japanese Patent Laid Open Application No. 40745/89. The multistage print head includes a plurality of head units as described above built upon each other for high print quality.
In these print heads, the drive coil is disposed on the nose side of the lever to utilize an attraction force for projecting a print wire through the nose end of the print head to effect printing. Because the drive coil is interposed between the nose end of the print head and the lever, the print wire must become unnecessarily long resulting in a corresponding increase in mass, making high speed driving difficult. Additionally, the size of the drive coil must be determined to provide the print wire with a predetermined impact force thus limiting the degree to which the print head may be miniaturized.
The prior art print head was constructed so that a print lever includes a shaft which is supported on a dish-shaped yoke of the driving coil. A damper is provided on a push plate positioned opposite to the drive coil to provide a top oscillation limit for the print lever. Therefore, it becomes difficult to precisely control a gap between a magnetic pole and print lever and the lever stroke cannot be kept constant. Furthermore, the prior art printer utilizes a spring holding member to maintain a coil spring therein for biasing an armature member of the print lever upwards. Therefore, the shape of the holding member becomes quite complicated. Additionally, if the bottom of the spring holding member hole becomes worn out through prolonged use, coil springs will vary in height from one to the other causing diverging outputs. In the multihead embodiment, a highly rigid retainer plate is interposed between adjacent head units. Therefore, the print wire of the farther head units becomes longer causing a disparity in printing operation between units.
Accordingly, it is desired to provide a wire dot print head which overcomes the shortcomings of the prior art.